Formation swapping - Comic about the TanDEM-X mission

An exciting manoeuvre awaits us. In early August (6–8 August 2013), the two TanDEM-X mission satellites will be reversing their formation. Until now, the TanDEM-X satellite has been circling around its twin, TerraSAR-X, in an anti-clockwise direction; after the reversal, it will circle clockwise. This complicated change to the formation in which they have been flying for almost three years is necessary to observe regions that are difficult to image, such as mountain ranges, from the opposite viewing angle. This blog entry takes an unconventional look at the planned change in formation ...

After a lengthy break, we want to liven up the TanDEM-X blog again with some up-to-date information regarding the mission. Although we have been operating the formation flight for three years now, it can hardly be thought of as routine. In all the institutes involved, people are continuing to work on planning the imaging, operating the instruments and satellites, and assessing the vast quantities of data with the same level of enthusiasm and effort as ever.

The second global coverage campaign for acquiring a Digital Elevation Model (DEM) of Earth was completed on 27 March 2013. The occasional observation gaps are currently being filled in, and Antarctica has now been completely imaged for the first time. Following this, the 'satellite swap' will be carried out on 6 August 2013. This will involve the relative motion of TerraSAR-X and TanDEM-X being changed in such a way that DEM data can be acquired along descending paths in the northern hemisphere (and ascending paths in the southern hemisphere). This complicated change to the formation in which they have been flying for almost three years is necessary to observe regions that are difficult to image, such as mountain ranges, from the opposite viewing angle.

During the formation swap, the satellites might come very close to one another. To minimise any risks for the mission, various safety measures are being employed. For one thing, no instruments will be operated during the three-day manoeuvre, to prevent mutual irradiation and hence potential damage to electronic components. If there were to be problems with the propulsion system during the swap, the worst-case scenario would be that the spatial separation of the satellites could be reduced to zero, resulting in a collision. Therefore, the formation swap will be performed when the satellites are 10 kilometres apart.

When planning the required orbital manoeuvre, I had the idea of illustrating the move in the form of a comic. Naturally, this is grossly exaggerated and divorced from physical constraints. As the comic was very popular with both my children and the TanDEM-X team (I got much more feedback than with scientific publications), I wanted to publish it here in this blog. I hope you enjoy reading it!

I would still like to emphasise that we can only allow ourselves to indulge in this playful treatment of the difficult operation we have planned as a result of the experience we have acquired in recent years running this unique mission and of the reliability of the two satellites. This continues to be an exciting mission. Keep your fingers crossed for the swap ...

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Ralph Kahle is the flight dynamics engineer for the TerraSAR-X / TanDEM-X Mission. His responsibilities include precise orbit and attitude determination for the satellites and measuring their position relative to one another, as well as the generation of auxiliary products for radar processing.
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